1. Field of the Invention
The present invention relates to a wavelength conversion apparatus capable of converting a wavelength of input light to another wavelength.
2. Description of the Related Art
In the field of communications using optical fibers, large-capacity and high-speed data transmission is required. Particularly, wavelength division multiplexing (WDM) and optical time division multiplexing (OTDM) are considered promising in that the transmission capacity of the optical fibers can be significantly increased, and a wavelength control technology for precisely controlling a plurality of carrier wavelengths and a wavelength conversion technology for converting a carrier wavelength to another carrier wavelength are important.
For example, in existing optical communication networks, single-wavelength optical transmission is mainstream that uses as the carrier wavelength the 1.3-xcexcm band in which the loss of optical fibers is small. Networks of this type are generally constructed for the purpose of substituting for telephone communication networks within cities. In trunk optical communication networks connecting cities, wavelength division multiplexing optical transmission is mainstream that uses as the carrier wavelength the 1.5-xcexcm band suitable for wavelength division multiplexing transmission.
When these types of optical communication networks are connected, since the carrier wavelengths are different from each other, it is necessary to temporarily convert the optical signals flowing on one network to electric signals and then convert the electric signals to optical signals using the carrier wavelength conforming to the other network. Then, the optical communication performance is limited according to the electric signal processing capability.
Therefore, by making it possible to directly convert the carrier wavelength of one network to the carrier wavelength of the other network, no electric signal processing is involved, so that the high performance of the optical communication can be effectively maintained. To do this, a light mixing technology for converting carrier wavelengths is essential.
In such wavelength conversion, since second harmonic generation (SHG), sum frequency generation (SFG), difference frequency generation (DFG), parametric conversion or the like by a nonlinear optical effect is used, a material with a high nonlinear optical effect is desired.
Examples of the related conventional art include Japanese Unexamined Patent Publication JP-A 10-213826 (1998), Japanese Unexamined Patent Publication JP-A 2000-10130 (2000), and a literature (IEICE Trans. Electron. Vol. E83-C, No. 6, pp. 869-874 (2000)).
The nonlinear optical effect largely depends on the polarization condition of the input light and the bearing of the nonlinear optical material. For example, when a light linearly polarized in a predetermined direction passes through an optical fiber, since the light is affected by the dispersion and the like of the optical fiber, the polarization condition at the exit of the optical fiber generally cannot be identified.
Moreover, since nonlinear optical materials are generally polarized light dependent in connection with wavelength conversion, when the polarization condition of the input light varies among carrier wavelengths, the wavelength conversion efficiency is inconstant, so that the intensity of the wavelength-converted output light is unstable.
An object of the invention is to provide a wavelength conversion apparatus capable of realizing stable wavelength conversion not depending on polarization conditions of signal light.
The invention relates to a wavelength conversion apparatus comprising:
a wavelength conversion element for carrying out wavelength conversion of a linearly polarized light component in a first direction; and
polarization rotating means including a reflecting element for reflecting light having passed through the wavelength conversion element to return the light to the wavelength conversion element, for making a polarization direction difference of 90 degrees between a light traveling from the wavelength conversion element to the reflecting element and a light reflected at the reflecting element so as to return to the wavelength conversion element.
According to the invention, the wavelength conversion element is polarization dependent, namely, the wavelength conversion element can wavelength-convert the linearly polarized light component in the first direction, but cannot wavelength-convert the linearly polarized light component in a second direction perpendicular to the first direction, and by disposing the polarization rotating means behind the wavelength conversion element, when a pumped light and a signal light are input, the first-direction components of the pumped light and signal light are wavelength-converted by the wavelength conversion element, and the second-direction components perpendicular to the first direction are not wavelength-converted.
Then, when the lights are reflected at the polarization rotating means, the first-direction components which are perpendicular to the first direction of the wavelength conversion element are not wavelength-converted, and the second-direction components which are parallel to the first direction of the wavelength conversion element are wavelength-converted. Consequently, the first-direction components are wavelength-converted on the way to the reflecting element and the second-direction components are wavelength-converted on the way back, so that the composite intensity of the wavelength-converted outputs is constant. As a result, stable wavelength conversion not depending on the polarization condition of the signal light can be realized.
Moreover, in the invention, it is preferable that the wavelength conversion apparatus comprises a wavelength selection reflecting element, disposed between the wavelength conversion element and the polarization rotating means, for selectively reflecting a pumped light.
According to the invention, by disposing the wavelength selection reflecting element for selectively reflecting the pumped light between the wavelength conversion element and the polarization rotating means, when the pumped light and the signal light are input, the first-direction components of the pumped light and signal light are wavelength-converted by the wavelength conversion element, and the second-direction components perpendicular to the first direction are not wavelength-converted. Then, only the pumped light is returned to the wavelength conversion element by the wavelength selection reflecting element and the signal light passes through the wavelength selection reflecting element as it is.
Then, after being reflected at the polarization rotating means, the first-direction component and the second-direction component are again rotated by 45 degrees about the optical axis by a 45-degree polarization rotating element, and pass through the wavelength selection reflecting element as they are. Then, the first-direction component of the signal light perpendicular to the first direction of the wavelength conversion element is not wavelength-converted, the second-direction component of the signal light parallel to the first direction of the wavelength conversion element is wavelength-converted, and at this time, the first-direction component of the pumped light contributes to wavelength conversion. Consequently, since the first-direction component of the pumped light can contribute to wavelength conversion on both ways, the wavelength conversion efficiency is improved. Preferably, by using a linearly polarized light in the first direction as the pumped light, the wavelength conversion efficiency can be further improved.
Moreover, the invention relates to a wavelength conversion apparatus comprising:
a polarization separation element made of a birefringent material, for separating, by beam walk-off, a light to be wavelength-converted into two linearly polarized light components perpendicular to each other so as to travel along two different optical axes;
a wavelength conversion element for carrying out wavelength conversion in accordance with the linearly polarized light components traveling along the two optical axes; and
a polarization multiplexing element for multiplexing two lights having passed through the wavelength conversion element.
According to the invention, after a light to be wavelength-converted is separated into two linearly polarized light components by beam walk-off, wavelength conversion for each of the two linearly polarized light components is carried out, and composition of the wavelength-converted components is carried out again, so that the composite intensity of the wavelength-converted output lights is constant. As a result, stable wavelength conversion not depending on the polarization condition of the signal light can be realized.
Moreover, the invention relates to a wavelength conversion apparatus comprising:
a polarization separation element for separating an incident light into a linearly polarized light in a first direction and a linearly polarized light in a second direction perpendicular to the first direction so that the linearly polarized light in the first direction travels along a first optical axis and the linearly polarized light in the second direction travels along a second optical axis;
a first 90-degree polarization rotating element for rotating by 90 degrees a polarization direction of a light traveling from the polarization separation element along the second optical axis;
a wavelength conversion element for wavelength-converting linearly polarized light components in the first direction with respect to a light traveling from the polarization separation element along the first optical axis and a light traveling from the first 90-degree polarization rotating element along the second optical axis;
a second 90-degree polarization rotating element for rotating by 90 degrees a polarization direction of a light traveling from the wavelength conversion element along the first optical axis; and
a polarization multiplexing element for multiplexing a light traveling from the second 90-degree polarization rotating element along the first optical axis and a light traveling from the wavelength conversion element along the second optical axis.
According to the invention, the wavelength conversion element is polarization dependent, namely, the wavelength conversion element can wavelength-convert the linearly polarized light component in the first direction, but cannot wavelength-convert the linearly polarized light component in the second direction. When the pumped light and the signal light are input to the polarization separation element disposed in front of the wavelength conversion element, the polarization separation element separates along the first optical axis the linearly polarized lights in the first direction with respect to the pumped light and the signal light and along the second optical axis the linearly polarized lights in the second direction perpendicular to the first direction. The pumped light and the signal light traveling along the first optical axis are wavelength-converted by the wavelength conversion element. As for the pumped light and the signal light traveling along the second optical axis, when the pumped light and the signal light pass through the first 90-degree polarization rotating element, the polarization directions thereof become parallel to the first direction, and the pumped light and the signal light are wavelength-converted by the wavelength conversion element.
As for the pumped light and the signal light traveling from the wavelength conversion element along the first optical axis, when the pumped light and the signal light pass through the second 90-degree polarization rotating element, the polarization directions thereof become parallel to the second direction, and the pumped light and the signal light enter the polarization multiplexing element. The pumped light and the signal light traveling from the wavelength conversion element along the second optical axis enter the polarization multiplexing element as they are, and the lights along the optical axes are multiplexed. Consequently, the first-direction component is wavelength-converted on the first optical axis and the second-direction component is wavelength-converted on the second optical axis, so that the composite intensity of the wavelength-converted output lights is constant. As a result, stable wavelength conversion not depending on the polarization condition of the signal light can be realized.
Moreover, the invention relates to a wavelength conversion apparatus comprising:
a polarization separation element for separating an incident light into a linearly polarized light in a first direction and a linearly polarized light in a second direction perpendicular to the first direction so that the linearly polarized light in the first direction travels along a first optical axis and the linearly polarized light in the second direction travels along a second optical axis;
a first wavelength conversion element for wavelength-converting a linearly polarized light component in the first direction, with respect to a light traveling from the polarization separation element along the first optical axis;
a second wavelength conversion element for wavelength-converting a linearly polarized light component in the second direction, with respect to a light traveling from the polarization separation element along the second optical axis; and
a polarization multiplexing element for multiplexing a light having passed through the first wavelength conversion element and a light having passed through the second wavelength conversion element.
According to the invention, the first and second wavelength conversion elements are polarization dependent, namely, the first wavelength conversion element can wavelength-convert the linearly polarized light component in the first direction, but cannot wavelength-convert the linearly polarized light component in the second direction, and the second wavelength conversion element can wavelength-convert the linearly polarized light component in the second direction, but cannot wavelength-convert the linearly polarized light component in the first direction. When the pumped light and the signal light are input to the polarization separation element disposed in front of the wavelength conversion elements, the polarization separation element separates the pumped light and the signal light into the linearly polarized lights in the first direction and the linearly polarized lights in the second direction perpendicular to the first direction so that the linearly polarized lights in the first direction travel along the first optical axis and the linearly polarized lights in the second direction travel along the second optical axis. The pumped light and the signal light traveling along the first optical axis are wavelength-converted by the first wavelength conversion element. The pumped light and the signal light traveling along the second optical axis are wavelength-converted by the second wavelength conversion element.
The pumped lights and the signal lights traveling from the first wavelength conversion element along the first optical axis and traveling from the second wavelength conversion element along the second optical axis are multiplexed by the polarization multiplexing element. Consequently, the first-direction component is wavelength-converted by the first wavelength conversion element, and the second-direction component is wavelength-converted by the second wavelength conversion element, so that the composite intensity of the wavelength-converted output lights is constant. As a result, stable wavelength conversion not depending on the polarization condition of the signal light can be realized.
Moreover, the invention relates to a wavelength conversion apparatus comprising:
a polarization separation element for separating an incident light into a linearly polarized light in a first direction and a linearly polarized light in a second direction perpendicular to the first direction so that the linearly polarized light in the first direction travels along a first optical axis and the linearly polarized light in the second direction travels along a second optical axis;
a 90-degree polarization rotating element for rotating by 90 degrees a polarization direction of a light traveling from the polarization separation element along the second optical axis;
a wavelength conversion element for wavelength-converting linearly polarized light components in the first direction, with respect to a light traveling from the polarization separation element along the first optical axis and a light traveling from the 90-degree polarization rotating element along the second optical axis; and
a reflecting element for reflecting lights traveling from the wavelength conversion element along the first optical axis and the second optical axis so that the light along the first optical axis passes through the wavelength conversion element, the light along the second optical axis passes through the wavelength conversion element and the 90-degree polarization rotating element in this order and these light are multiplexed by the polarization separation element.
According to the invention, the wavelength conversion element is polarization dependent, namely, the wavelength conversion element can wavelength-convert the linearly polarized light component in the first direction, but cannot wavelength-convert the linearly polarized light component in the second direction. When the pumped light and the signal light are input to the polarization separation element disposed in front of the wavelength conversion element, the polarization separation element separates along the first optical axis the linearly polarized lights in the first direction, with respect to the pumped light and the signal light and separates along the second optical axis the linearly polarized lights in the second direction perpendicular to the first direction. The pumped light and the signal light traveling along the first optical axis are wavelength-converted by the wavelength conversion element. As for the pumped light and the signal light traveling along the second optical axis, when the pumped light and the signal light pass through the first 90-degree polarization rotating element, the polarization directions thereof become parallel to the first direction, and the pumped light and the signal light are wavelength-converted by the wavelength conversion element.
The pumped lights, the signal lights and the output lights traveling from the wavelength conversion element along the first optical axis and the second optical axis are reflected at the reflecting element, and then, again pass through the same wavelength conversion element to be wavelength-converted. The pumped light, the signal light and the output light traveling along the second optical axis pass through the 90-degree polarization rotating element, so that the polarization directions thereof are made parallel to the second direction. Then, the lights enter the polarization separation element. The pumped lights, the signal lights and the output lights traveling along the optical axes are multiplexed by the polarization separation element. Consequently, the first-direction component is wavelength-converted on the first optical axis and the second-direction component is wavelength-converted on the second optical axis, so that the composite intensity of the wavelength-converted output lights is constant. As a result, stable wavelength conversion not depending on the polarization condition of the signal light can be realized.
Moreover, the invention relates to a wavelength conversion apparatus comprising:
a polarization separation element for separating an incident light into a linearly polarized light in a first direction and a linearly polarized light in a second direction perpendicular to the first direction so that the linearly polarized light in the first direction travels along a first optical axis and the linearly polarized light in the second direction travels along a second optical axis;
a first wavelength conversion element for wavelength-converting a linearly polarized light component in the first direction, with respect to a light traveling from the polarization separation element along the first optical axis;
a second wavelength conversion element for wavelength-converting a linearly polarized light component in the second direction, with respect to a light traveling from the polarization separation element along the second optical axis; and
a reflecting element for reflecting a light traveling from the first wavelength conversion element along the first optical axis and a light traveling from the second wavelength conversion element along the second optical axis so that a light having passed through the first wavelength conversion element and a light having passed through the second wavelength conversion element are multiplexed by the polarization separation element.
According to the invention, the first and second wavelength conversion elements are polarization dependent, namely, the first wavelength conversion element can wavelength-convert the linearly polarized light component in the first direction, but cannot wavelength-convert the linearly polarized light component in the second direction, and the second wavelength conversion element can wavelength-convert the linearly polarized light component in the second direction, but cannot wavelength-convert the linearly polarized light in the first direction. When the pumped light and the signal light are input to the polarization separation element disposed in front of the wavelength conversion elements, the polarization separation element separates along the first optical axis the linearly polarized lights in the first direction, with respect to the pumped light and the signal light, and along the second optical axis the linearly polarized lights in the second direction perpendicular to the first direction. The pumped light and the signal light traveling along the first optical axis are wavelength-converted by the first wavelength conversion element. The pumped light and the signal light traveling along the second optical axis are wavelength-converted by the second wavelength conversion element.
The pumped lights and the signal lights traveling from the wavelength conversion elements along the first optical axis and the second optical axis are reflected at the reflecting element, again pass through the same wavelength conversion elements to be wavelength-converted, and then, are multiplexed by the polarization multiplexing element. Consequently, the first-direction component is wavelength-converted by the first wavelength conversion element and the second-direction component is wavelength-converted by the second wavelength conversion element, so that the composite intensity of the wavelength-converted output lights is constant. As a result, stable wavelength conversion not depending on the polarization condition of the signal light can be realized.
Moreover, the invention relates to a wavelength conversion apparatus comprising:
a polarization separation element for separating an incident light into a linearly polarized light in a first direction and a linearly polarized light in a second direction perpendicular to the first direction so that the linearly polarized light in the first direction travels along a first optical axis and the linearly polarized light in the second direction travels along a second optical axis;
a wavelength conversion element for wavelength-converting a linearly polarized light component in the first direction, with respect to a light traveling from the polarization separation element along the first optical axis;
a polarization multiplexing element for multiplexing a light traveling from the wavelength conversion element along the first optical axis and a light traveling from the polarization separation element along the second optical axis; and
polarization rotating means including a reflecting element for reflecting a light having passed through the polarization multiplexing element to return the light to the wavelength conversion element, for making a polarization direction difference of 90 degrees between a light traveling from the wavelength conversion element to the reflecting element and a light reflected at the reflecting element so as to return to the wavelength conversion element.
According to the invention, the wavelength conversion element is polarization dependent, namely, the wavelength conversion element can wavelength-convert the linearly polarized light component in the first direction, but cannot wavelength-convert the linearly polarized light component in the second direction. When the pumped light and the signal light are input to the polarization separation element disposed in front of the wavelength conversion element, the polarization separation element separates along the first optical axis the linearly polarized lights in the first direction, with respect to the pumped light and the signal light, and along the second optical axis the linearly polarized lights in the second direction perpendicular to the first direction. The pumped light and the signal light traveling along the first optical axis are wavelength-converted by the wavelength conversion element. The pumped light and the signal light traveling along the second optical axis are not wavelength-converted. The pumped lights and the signal lights traveling from the wavelength conversion element along the first optical axis and the second optical axis are multiplexed by the polarization multiplexing element.
Then, when the lights are reflected at the polarization rotating means, the polarization directions of the lights are rotated by 90 degrees. Then, the polarization multiplexing element separates along the second optical axis the pumped light and the signal light traveling along the first optical axis on the way to the reflecting element, and separates along the first optical axis the pumped light and the signal traveling along the second optical axis on the way back. The lights along the first optical axis are wavelength-converted by the wavelength conversion element, whereas the lights along the second optical axis are not wavelength-converted. Then, the pumped lights and the signal lights traveling from the wavelength conversion element along the first optical axis and the second optical axis are multiplexed by the polarization separation element. Consequently, the first-direction component is wavelength-converted on the way to the reflecting element and the second-direction component is wavelength-converted on the way back, so that the composite intensity of the wavelength-converted output lights is constant. As a result, stable wavelength conversion not depending on the polarization condition of the signal light can be realized.
Moreover, the invention relates to a wavelength conversion apparatus comprising:
a polarization separation element for separating an incident light into a linearly polarized light in a first direction and a linearly polarized light in a second direction perpendicular to the first direction so that the linearly polarized light in the first direction travels along a first optical axis and the linearly polarized light in the second direction travels along a second optical axis;
a first 90-degree polarization rotating element for rotating by 90 degrees a polarization direction of a light traveling from the polarization separation element along the second optical axis;
a wavelength conversion element for wavelength-converting linearly polarized light components in the first direction, with respect to a light traveling from the polarization separation element along the first optical axis and a light traveling from the first 90-degree polarization rotating element along the second optical axis;
a second 90-degree polarization rotating element for rotating by 90 degrees a polarization direction of a light traveling from the wavelength conversion element along the first optical axis;
a polarization multiplexing element for multiplexing a light traveling from the second 90-degree polarization rotating element along the first optical axis and a light traveling from the wavelength conversion element along the second optical axis; and
polarization rotating means including a reflecting element for reflecting a light having passed through the polarization multiplexing element to return the light to the wavelength conversion element, for making a polarization direction difference of 90 degrees between a light traveling from the wavelength conversion element to the reflecting element and a light reflected at the reflecting element so as to return to the wavelength conversion element.
According to the invention, the wavelength conversion element is polarization dependent, namely, the wavelength conversion element can wavelength-convert the linearly polarized light component in the first direction, but cannot wavelength-convert the linearly polarized light component in the second direction. When the pumped light and the signal light are input to the polarization separation element disposed in front of the wavelength conversion element, the polarization separation element separates along the first optical axis the linearly polarized lights in the first direction, with respect to the pumped light and the signal light, and along the second optical axis the linearly polarized lights in the second direction perpendicular to the first direction. The pumped light and the signal light traveling along the first optical axis are wavelength-converted by the wavelength conversion element. As for the pumped light and the signal light traveling along the second optical axis, when the pumped light and the signal light pass through the first 90-degree polarization rotating element, the polarization directions thereof become parallel to the first direction, and the pumped light and the signal light are wavelength-converted by the wavelength conversion element.
The pumped light and the signal light traveling from the wavelength conversion element along the first optical axis pass through the second 90-degree polarization rotating element, so that the polarization directions thereof are made parallel to the second direction. Then, the lights enter the polarization multiplexing element. The pumped light and the signal light traveling from the wavelength conversion element along the second optical axis enter the polarization multiplexing element as they are. The lights along the optical axes are multiplexed.
Then, when the lights are reflected at the polarization rotating means, the polarization directions of the lights are rotated by 90 degrees. Then, the polarization multiplexing element separates along the second optical axis the pumped light and the signal light traveling along the first optical axis on the way to the reflecting element, and separates along the first optical axis the pumped light and the signal light traveling along the second optical axis on the way back. The pumped light and the signal light traveling along the first optical axis are wavelength-converted by the wavelength conversion element, and enters the polarization separation element as they are. As for the pumped light and the signal light traveling along the second optical axis, when the pumped light and the signal light pass through the second 90-degree polarization rotating element, the polarization directions thereof become parallel to the first direction, and the pumped light and the signal light are wavelength-converted by the wavelength conversion element. Then, when the pumped light and the signal light pass through the first 90-degree polarization rotating element, the polarization directions thereof become parallel to the second direction, and the pumped light and the signal light enter the polarization separation element. The pumped lights and the signal lights traveling along the optical axes are multiplexed by the polarization separation element. Consequently, the first-direction component and the second-direction component are wavelength-converted on both ways, so that the composite intensity of the wavelength-converted output lights is constant. As a result, stable wavelength conversion not depending on the polarization condition of the signal light can be realized.
Moreover, the invention relates to a wavelength conversion apparatus comprising:
a polarization separation element for separating an incident light into a linearly polarized light in a first direction and a linearly polarized light in a second direction perpendicular to the first direction so that the linearly polarized light in the first direction travels along a first optical axis and the linearly polarized light in the second direction travels along a second optical axis;
a first wavelength conversion element for wavelength-converting a linearly polarized light component in the first direction, with respect to a light traveling from the polarization separation element along the first optical axis;
a second wavelength conversion element for wavelength-converting a linearly polarized light component in the second direction, with respect to a light traveling from the polarization separation element along the second optical axis;
a polarization multiplexing element for multiplexing a light traveling from the first wavelength conversion element along the first optical axis and a light traveling from the second wavelength conversion element along the second optical axis; and
polarization rotating means including a reflecting element for reflecting a light having passed through the polarization multiplexing element to return the light to the wavelength conversion element, for making a polarization direction difference of 90 degrees between a light traveling from the wavelength conversion element to the reflecting element and a light reflected at the reflecting element so as to return to the wavelength conversion element.
According to the invention, the first and second wavelength conversion elements are polarization dependent, namely, the first wavelength conversion element can wavelength-convert the linearly polarized light component in the first direction, but cannot wavelength-convert the linearly polarized light component in the second direction, and the second wavelength conversion element can wavelength-convert the linearly polarized light component in the second direction, but cannot wavelength-convert the linearly polarized light component in the first direction. When the pumped light and the signal light are input to the polarization separation element disposed in front of the wavelength conversion elements, the polarization separation element separates along the first optical axis the linearly polarized lights in the first direction, with respect to the pumped light and the signal light, and along the second optical axis the linearly polarized lights in the second direction perpendicular to the first direction. The pumped light and the signal light traveling along the first optical axis are wavelength-converted by the first wavelength conversion element. The pumped light and the signal light traveling along the second optical axis are wavelength-converted by the second wavelength conversion element.
The pumped lights and the signal lights traveling from the first wavelength conversion element along the first optical axis and traveling from the second wavelength conversion element along the second optical axis are multiplexed by the polarization multiplexing element.
Then, when the lights are reflected at the polarization rotating means, the polarization directions of the lights are rotated by 90 degrees. Then, the polarization multiplexing element separates along the second optical axis the pumped light and the signal light traveling along the first optical axis on the way to the reflecting element, and along the first optical axis the pumped light and the signal light traveling along the second optical axis on the way back. The light along the first optical axis is wavelength-converted by the first wavelength conversion element, and the light along the second optical axis is wavelength-converted by the second wavelength conversion element.
The pumped lights and the signal lights traveling from the first wavelength conversion element along the first optical axis and traveling from the second wavelength conversion element along the second optical axis are multiplexed by the polarization multiplexing element. Consequently, the first-direction component and the second-direction component are wavelength-converted on both ways, so that the composite intensity of the wavelength-converted output lights is constant. As a result, stable wavelength conversion not depending on the polarization condition of the signal light can be realized.
Moreover, in the invention, it is preferable that the polarization rotating means comprises a 45-degree rotating element for rotating a polarization direction of light by 45 degrees and the reflecting element for reflecting a light having passed through the 45-degree polarization rotating element to return the light to the 45-degree polarization rotating element, and the 45-degree rotating element is disposed between the wavelength conversion element and the reflecting element.
According to the invention, the light incident on the polarization rotating means can be returned to the wavelength conversion element by rotating the polarization direction of the light by 90 degrees.
Moreover, in the invention, it is preferable that the polarization rotating means comprises a xcex/4 plate disposed between the reflecting element and the wavelength conversion element, and the reflecting element.
According to the invention, the light incident on the polarization rotating means can be returned to the wavelength conversion element by rotating the polarization direction of the light by 90 degrees.
As described above in detail, according to the invention, by wavelength conversion with respect to the linearly polarized light component in the first direction and the linearly polarized light component in the second direction perpendicular to the first direction, the composite intensity of the wavelength-converted output lights is constant, so that stable wavelength conversion not depending on the polarization condition of the signal light can be realized.